An optical limiting amplifier is an optical amplifier that yields an output power which is insensitive to the variation of the input power. The gain of an optical limiting amplifier varies with the input power, and the response time is characterized by the gain recovery time of the optical amplifier. Examples of optical limiting amplifiers include deeply saturated erbium-doped fiber amplifier (EDFA) and deeply saturated semiconductor optical amplifier (SOA). The gain recovery time of a saturated EDFA is usually on the order of a millisecond, whereas the gain recovery time of a saturated SOA ranges from picoseconds to nanoseconds. Depending on the gain recovery time, the optical limiting amplifier may find a wide range of potential applications in optical communication systems. It can be used as a power equalizer at the optical transmitter to regulate the launch power; it can be used at optical add-drop nodes to suppress transient optical power fluctuations; it can be used as an optical preamplifier in front of a receiver to increase the receiver's power dynamic range; it can also be used to erase amplitude-shift keyed (ASK) optical labels on optically labeled signals. Since the gain dynamics of the EDFA is very slow, it is only good for suppressing slow power fluctuations at its input and not suitable for fast power equalization. The SOA, on the other hand, has much faster gain dynamics and is capable of regulating the optical power on a much smaller time scale. However, the use of a deeply saturated SOA as a power-limiting amplifier has been considered not practical because the gain recovery time of the SOA is comparable to the bit period and hence strong waveform distortion occurs when the SOA operates in the deep saturation regime with the conventional on-off keying (OOK) optical signals.
Novel modulation schemes have been considered to improve the performance of saturated SOAs. Recently, significant reduction of cross-gain modulation in the SOA was successfully demonstrated for wavelength-division multiplexed (WDM) transmission with return-to-zero differential phase-shift keying (RZ-DPSK), see P. S. Cho and J. B. Khurgin, “Suppression of cross-gain modulation in SOA using RZ-DPSK modulation format,” IEEE Photon. Technol. Lett., vol. 15, pp. 162-164, 2003. However, no prior art schemes have proposed employing a deeply saturated SOA as an optical power limiting amplifier to rapidly equalize the power of an optical signal.